This invention relates to filter circuits, and in particular, to a switched capacitor filter circuit which closely matches an RC filter circuit.
Filters employing thin film resistor and capacitor combinations are quite advantageous in a number of applications, especially as high pass filters in telecommunications systems. Such a simple RC filter is illustrated in FIG. 1. For such a circuit, it is known that the filter pole f.sub.p will be determined by the relationship: ##EQU1## where R is the resistance of the resistor, and C is the capacitance of the capacitor.
Such filters can provide extremely high accuracy. However, it would be more economical if the filter could be integrated onto a semiconductor chip. Unfortunately, it is difficult to fabricate a resistor having the accuracy required for precision filters with present integrated circuit technology. Capacitors can be fabricated with the necessary accuracy, however, and it has therefore been suggested that a switched capacitor could simulate the function of a resistor. Thus, a sampled-data filter integrated circuit which matched the RC filter response could be achieved. (See, for example, Brodersen et al, "MOS Switched-Capacitor Filters", Proceedings Of The IEEE, Vol. 67, No. 1, pp. 61-75 (January 1979)). Various designs of switched capacitor filters involving various approximations have been proposed as an RC filter equivalent (see, e.g., Fried, "Analog Sample-Data Filters", IEEE Journal Of Solid-State Circuits, Vol. SC-7, pp. 302-304 (August 1972)). Most prior art suggestions do not provide a close match to the RC circuit for desirably low switching frequencies (e.g., 30 KHz) where the input frequency is above 1 KHz.
It is therefore a primary object of the invention to provide a switched capacitor filter circuit which closely matches the frequency response of an RC filter circuit without significant insertion loss.